The so-called urethane tar, which forms a satisfactory waterproof coating membrane, has been used as a protective coating for wood, concrete and other materials for a long time. However, not only the urethane tar coating but polyurethane coating materials in general often cause bubbling when water is present in the articles to be coated or the coating instrument. The user must be extremely careful for existence of water and thick coating must be carefully avoided.
The conventional water-curable urethane materials are used for filling cracks in concrete structures or for grouting the ground or soil. But in this case, the urethane materials are cured with water in 2-5 minutes, and therefore, penetration or infiltration of the materials into the cracks or the soil is not sufficient enough to repair the cracks or to strengthen the soil.
We got the idea that if the curing time of the urethane material could be prolonged, carbon dioxide gas generated in the course of curing of the material would easily escape out of the hardening resin system and the resulting cured polymer would contain very few or no bubbles. So, we tested various urethane materials and we have found that some urethane prepolymers require a long time for curing and form hydrogel containing extremely few or no bubbles in the presence of water. The finding has led us to this invention.